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The Pliensbachian–Toarcian transition was characterised by a drastic turnover from a cool climate to a period of rapid global warming. While the warming associated with the Early Toarcian Oceanic Anoxic Event is rather well-studied, the cause, intensity and extent of the preceding cooling in the late Pliensbachian are still discussed. Occurrences of glendonite play an important role in this debate, since glendonite is a pseudomorph after the cryophilic carbonate mineral ikaite. This study describes the first glendonite-bearing carbonate concretions from South Germany (Buttenheim clay pit, northern Franconian Alb), which represent the southernmost glendonite occurrence in the late Pliensbachian documented so far. Based on petrographical and sedimentological investigations as well as stable isotope analyses it is concluded that a low temperature was the main factor for ikaite formation in the studied section, suggesting that the late Pliensbachian cooling had a more far-reaching impact on the temperature of the European epicontinental sea than previously assumed. To explain the low temperatures required for ikaite precipitation, a model for the sea-ice driven formation of cold bottom-water masses on the continental shelf is proposed. The occurrence of several layers containing reworked hiatus concretions in the studied outcrop is interpreted as the result of recurrent sea-level falls caused by multiple glacial pulses characterising the overall cool climate in the late Pliensbachian.

The Ordovician Chiatsun Group, exposed at the Jiacun section, Nyalam, in southern Xizang (Tibet), China, is biostratigraphically one of the best constrained Ordovician sections in the Himalaya margin, and thus plays a key role in correlating the Ordovician strata of this region. The Chiatsun Group is subdivided into the Adang Formation, the Alai Formation, and the Jiaqu Formation, in stratigraphically ascending order. Based on limestone samples collected from the Alai and Jiaqu formations, the Darriwilian chemostratigraphy is established and confirmed by conodont biozonation data. The Middle Darriwilian δ 13 C excursion (MDICE) is recorded in the Himalaya margin for the first time, with an amplitude of about 2‰. Its rising limb starts near the boundary of the Histiodella holodentata and Histiodella kristinae zones, and the excursion reaches its maximum values in the Pygodus serra Zone. The decreasing limb is not exposed. The carbon isotope curve with its pronounced onset of the MDICE documents a high potential for chemostratigraphic correlations helping to disentangle the Kurgiakh orogeny in the Himalaya margin. The MDICE recorded in southern Xizang correlates well with that in Baltoscandia, Laurentia, the Argentine Precordillera, Siberia, South China, North China, and Tarim, and extends the distribution of the MDICE to a new block, and thereby strengthens its global significance.

The platycerate gastropods Orthonychia yutaroi Ebbestad, sp. nov. (Ordovician, Boda Limestone, Sweden), O. enorme (Silurian, Sweden, Gotland), O. parva (Pennsylvanian, Finis Shale Member, USA), and Orthonychia sp. (Mississippian, Imo Formation, USA) are studied including their protoconch morphology. Orthonychia yutaroi is the oldest known species in Orthonychia . Platycerates contain species with both, openly and tightly coiled protoconchs. This is the first report that tightly coiled protoconchs occur in Orthonychia . This and previously published observations blur the diagnostic difference between orders Cyrtoneritimorpha (openly coiled protoconch) and Cycloneritimorpha (tightly coiled protoconch). We suggest to treat Cyrtoneritimorpha and Cycloneritimorpha as synonyms of Neritimorpha. The monotypic Devonian genus Pragoserpulina is morphological so close to the Orthonychia species reported herein that synonymy of both genera seems to be possible (and thus of the families Pragoserpulinidae and Orthonychiidae). Protoconch morphology and dimensions suggest that the studied platycerate species had planktotrophic larval development. By contrast, two studied Carboniferous euomphaloid species (one with an openly and the other with tightly coiled protoconch) have paucispiral, large protoconchs indicating non-planktotrophic larval development. We assume that openly and tightly coiled protoconchs were present in various Paleozoic gastropod clades and that selection acted against the openly coiled protoconch morphology. It has previously been proposed that increasing predation pressure in the plankton was the reason for the demise of openly coiled protoconchs (Paleozoic plankton revolution). The presence of larval planktotrophy in platycerates excludes the possibility that they belong to extant basal gastropod clades such as Patellogastropoda, Cocculiniformia, and Vetigastropoda. However, a previously proposed close relationship to Neritimorpha is corroborated.

Reefs in the Late Ordovician are usually built up by metazoans, whereas in the Early and Middle Ordovician microbial reefs are dominating. In the Late Ordovician (Katian) Lianglitag Formation, at Bachu, Tarim, NW China, however, three distinct stages of thrombolithic microbial reefs are exposed. The lower and upper reef intervals are characterized by widespread microbial carbonates with almost constant thicknesses (biostromes). The middle reef interval exposes dome-like mounds, and shows a higher diversity of reef-building and reef-dwelling organisms. Calcareous algae such as Vermiporella are abundant, but also other components such as fragments of Halysis , brachiopods, molluscs, echinoderms, bryozoans, and trilobites have been found in the microbial reef units. The purpose of this study is to describe for the first time the composition and microfacies of the reefs in this remote area. Especially the question of whether or not these Late Ordovician reefs represent remnants of the Early/Middle Ordovician microbial-dominated reef type just lacking Calathium and lithistid sponge is addressed. The results indicate that the local conditions on the leeward side of the carbonate platform, where waters are less well agitated and thus less well oxygenated, and probably also characterized by temporarily elevated water temperatures hampering the growth of metazoan reefs, were responsible for the proliferation of the Late Ordovician microbial reefs in the Bachu area.

Climatically the Silurian was a most unstable period, as it is shows several strong δ 13 C excursions, with the Lau excursion being the strongest short-lived positive δ 13 C excursion of the entire Phanerozoic. The causes of these excursions remain a topic of debate. In order to determine how benthic communities responded to these climatic perturbations, a microfacies analysis of carbonate rocks from the Silurian period (Late Telychian to the uppermost Ludfordian) of Gotland was conducted. The development of reefs was significantly enhanced during periods characterized by carbon isotope excursions, compared to time intervals without. In periods of elevated δ 13 C values (≥ 3 ‰) the abundance of bryozoans and microbes increases, and in some areas, they form reefs (bryostromatolites), while corals and stromatoporoids were the main reef builders in periods of lower isotope ratios (0–3 ‰). Furthermore, a significant increase of phosphatic components, gypsum pseudomorphs and the microproblematicum Palaeomicrocodium is observed in times of increased isotope ratios. A model is proposed that explains the changes in the benthic communities over time. It is based on previously published models and explains the geochemical processes from coastal areas to the open ocean. Changes are assumed from a humid climate in times without isotope excursions to an arid climate in times of isotope excursions. It emphasizes processes that may have led to the strong δ 13 C shifts during the Silurian period, and includes the phosphorus cycle in sea water as an important factor in the development of the benthic communities.

The Mulde Event (Homerian, Wenlock) of the Silurian is characterized by a positive [δ.sup.13]C isotope excursion, a stepwise extinction of the hemipelagic fauna and an increase in non-skeletal deposits thought to be microbial, proliferating in shallow marine environments. There is little known about whether the proliferation of microbialites is due to reduced grazing, increased seawater saturation state or an increase in nutrient supply. We have studied a Mulde Event-associated buildup from the Muksha Formation in Bagovytsya, Podolia, Western Ukraine. This buildup differs from a typical Silurian skeletal reef in its low abundance of rugose and favositid corals. The framework of this reef is dominated by stromatoporoids, heliolitid corals and calcimicrobes, the last ones encrusting the stromatoporoids and shells and also forming oncoids. Several microproblematica are present, including Girvanella, Rothpletzella and Hedstroemia, along with Ortonella-like porostromate problematica. This study attempts to further identify the Ortonella-like porostromate problematica and Hedstroemia through SEM analyses which have revealed a recrystallized structure with microdolomite, indicating an originally high-Mg calcite composition. This suggests a red algal affinity or possibly a microbial origin. Our observations may help to constrain the environmental controls on the development of microbial- and microproblematica-dominated deposits during the Mulde Event. Key words: microbialite, Hedstroemia, Silurian, calcimicrobes, reefs, carbon isotope excursion.

A new cornulitid species, Conchicolites crispisulcans sp. nov, is described from the Wenlock of Gotland, Sweden. The undulating edge of C. crispisulcans sp. nov. peristomes is unique among the species of Conchicolites. This undulating peristome edge may reflect the position of setae at the tube aperture. The presence of the undulating peristome edge supports the hypothesis that, cornulitids had setae and were probably related to brachiopods.

Limestone-marl alternations (LMA) are rhythmical successions of carbonate-rich sedimentary rocks. They are often assumed to record cyclic sedimentation linked to Milankovitch cycles. In spite of the importance of LMA for a range of questions in geosciences, it is not unequivocally understood how they originate. The two models explaining their origin both assume carbonate redistribution, either by late diagenetic pressure dissolution amplifying primary depositional differences, or through early diagenetic aragonite dissolution and reprecipitation as calcite, creating LMA even in the absence of primary differences. The latter model is known as differential diagenesis. As both models can imply different interpretations of paleoenvironmental conditions, the identification of the generating process is essential. This study addresses the question how to distinguish the generating process through statistical comparison of taphonomic characteristics of marls and limestones in thin sections by: (1) measuring the relative abundance of originally aragonitic and calcitic components in the fossil assemblages, and (2) by analysis of their orientations. Based on four sets of thin sections from different paleoenvironments from the Upper Ordovician to the Permian, the model of late diagenetic pressure-induced carbonate redistribution is ruled out. The results point towards early diagenetic aragonite dissolution and reprecipitation as calcite as the main diagenetic process generating LMA. Furthermore, the influence of primary sedimentary differences is demonstrated. This approach offers a tool to gauge conditions during sedimentation and a way to assess the systematically poorer preservation of aragonitic components in marine deposystems (aragonite bias) quantitatively.

Wetheredella is a calcareous encrusting microproblematicum known only from its two-dimensional expression in thin-sections. It occurs in Cambrian through Permian rocks, often in association with Rothpletzella and Girvanella , probable calcifying cyanobacteria. We show that Wetheredella can be produced by sectioning of the Paleozoic incertae sedis sclerobiont Allonema , which has until now been recognized based on its surface, 3D aspect. Therefore, we propose that Wetheredella should be regarded as a junior synonym of Allonema . Structures resembling Wetheredella have also been obtained in thin-sections through another encrusting microproblematicum, Ascodictyon , which, however, differs in the ultrastructure of its wall, which consists of parallel calcite crystals in contrast to radial crystals in Allonema . Allonema specimens used in the study were extracted from the Middle Silurian Mulde Brick-clay Member (Gotland, Sweden), representing an off-platform environment below the photic zone. This, together with the primary calcitic, radial wall structure of Allonema argues against a cyanobacterial or algal affinity, which had been considered for Wetheredella . The taxonomic position of Allonema remains unresolved, but the sclerobiont shares many common characteristics with some Paleozoic encrusting foraminifers. Combining the occurrence data on Allonema in its surface aspect and the “ Wetheredella ” aspect recognized in thin-sections may shed more light on its affinity and paleoecological functions, particularly its association with cyanobacteria and its role as a framework-builder in Paleozoic reefs.

A microbial origin has been proposed for matrix-supported, low-diversity buildups reported from different palaeocontinents during the onset of the Mulde positive carbon isotope excursion. We have investigated a small aphanitic buildup from the Lower Quarried Limestone Member of the Much Wenlock Limestone Formation, exposed at Whitman?s Hill (Herefordshire), corresponding to the central part of the Midland Platform (UK). Up to 50% of the rock volume in this buildup consists of mottled micrite. The SEM studies revealed that the micrite is largely detrital and does not show features characteristic of calcareous cyanobacteria or leiolites. The aphanitic character of the buildup is suggested to be controlled by the depositional rate, and the widespread occurrence of matrix-supported reefs in this interval to be driven by a mid-Homerian rapid eustatic transgression.